(a) Field of the Invention
The present invention relates to a pattern test device and, more particularly, to a pattern test device for detecting a defect in a test pattern data by comparing the test pattern data obtained by imaging a mask pattern against a reference pattern data. The present invention also relates to a pattern test method.
(b) Description of the Related Art
A typical test method for detecting a defect in a mask pattern or reticle pattern (simply referred to as mask pattern in this text) uses a test pattern data obtained by imaging the mask pattern in a mask by using a CCD camera or optical imaging system, for example. The test pattern data are compared against corresponding reference pattern data or design data stored in a CAD system for electron beam writing.
FIG. 5 shows an example of a conventional pattern test device. The pattern test device, generally designated by numeral 11, includes an imaging system 12, a conversion section 13, a comparator 15, a judgement section 16, and a review section 17 such as including a display unit.
The imaging system 12 picks-up an image from the mask pattern in a mask under test, to thereby generate test pattern data, and delivers the same to the comparator 15. The test pattern data includes a plurality of unit pixel data, which are encoded into gray-scale levels from 0 to 255 depending on the brightness thereof, for example.
The conversion section 13 receives electron beam data (EB data) stored in the CAD system corresponding to the mask pattern data, converts the EB data into reference pattern data having a data format same as the data format of the test pattern data, and delivers the converted EB data as reference pattern data to the comparator 15.
The comparator 15 compares the test pattern data delivered from the imaging system 12 against the reference pattern data delivered from the conversion section 13, to deliver differential data between the test pattern data and the reference pattern data to the judgement section 16. The term “differential data” as used herein include level difference data representing a difference between the gray-scale level of the test pattern data and the gray-scale level of the reference pattern data for each pixel, and differential difference data representing a difference between the differentiated data of the test pattern data with respect to the dimension in a specified direction and the differentiated data of the reference data with respect to the dimension in the specified direction.
The judgement section 16 stores therein threshold values used for judging as to whether or not the difference data exhibit a defect in the test pattern data. The judgement section 16 compares the difference data including the level reference data and the differential difference data against the respective threshold values, to judge presence or absence of the defect depending on the magnitude of the difference data with respect to the threshold values. The judgement section 16 stores therein the to coordinates of the defect thus judged and the image data of the vicinity of the defect. The judgement section 16 then delivers the coordinates and the image data of the defect to the review section 17.
The review section 17, or display unit, displays the image of the vicinity of the defect based on the coordinate information and the image data received from the judgement section 16. The operator observes the image on the review section 17 and determines the size and type of the defect as well as the influence thereby upon the resultant semiconductor device.
It is to be noted that different defects in the mask pattern have different influences on the resultant LSI (or semiconductor device) depending on the location of the defects in the mask pattern even if the defects have similar sizes. However, this is not noticed in the conventional technique, and a minor defect which has an insignificant influence on the resultant semiconductor device is also detected by the conventional technique. This reduces the throughput of the pattern test device and raises a turn around time for fabricating the semiconductor device.
Patent Publication JP-A-2000-146857 describes a technique for solving the above problem, wherein the threshold for detecting the defect is changed depending on the sub-areas of the mask. FIG. 6 shows the pattern test device described in the publication. The pattern test device 50 includes a host CPU 59, a storage disk 60, a design data input section 61, a data comparator 62 and an imaging system 52.
The imaging system 52 includes a light source 54, an illumination optical system 55, an X-Y table for mounting thereon a photomask under test, an image forming optical system 56, a photodetector 57 and a sensor circuit 58. The sensor circuit 58 has a function for encoding the output from the photodetector 57.
The storage disk 60 stores therein CAD data including design pattern data of the photomask 51. The host CPU 59 reads the CAD data from the storage disk 60, and the design data input section 61 receives the design pattern data in the CAD data from the host CPU 59 to deliver the same as the reference pattern data to the data comparator 62.
The imaging system 52 detects the pattern of the photomask 51 by using the function of the optical systems 55 and 56 and the photodetector 57, and delivers the test pattern data representing the imaged patterns of the photomask 51 to the data comparator 62 through the sensor circuit 58.
FIG. 7 shows the explanatory diagram for showing the principle of the pattern test device of FIG. 6, wherein the thresholds are changed for the sub-areas in the mask area 63 of the photomask 51. In the pattern test device 50, the host CPU 59 delivers, to the data comparator 62, data for dividing the mask area 63 of the photomask 51 into sub-areas A, B and C for which respective thresholds are determined beforehand.
FIG. 8 shows the functional block diagram of the data comparator 62, which includes a differential comparator block 70 and a level comparator 71. The differential comparator block 70 includes a differential circuit 77, an edge orientation detector 72, an edge differentiation circuit 73, a selector 74, a maximum detector 75 and a subtracter 76.
The differentiation circuit 77 differentiates the test pattern data with respect to the orientations of X-axis, Y-axis and orientations ±45 degrees away from the X-axis, and delivers the absolute values of the differentials to the selector 74. The edge orientation detector 72 detects the orientation of an edge of a pattern based on the design pattern data. The edge differentiation circuit 73 differentiates the pixel data with respect to the orientation of the edge of the pattern received from the edge orientation detector 72. The maximum detector 75 detects the maximum absolute value among the differentials of the pixel data around the pattern.
The selector 74 selects one of the absolute values of the differentials which is differentiated with respect to an orientation same as the orientation of the edge of the pattern detected by the edge orientation detector 72.
The subtracter 76 subtracts the maximum differential detected by the maximum detector 75 from the differential selected by the selector 74, and delivers the differential difference to the judgement section 78.
The judgement section 78 compares the differential difference delivered from the subtracter 76 against the threshold, which is predetermined for the sub-area of the mask, to thereby detects a defect. The information of the defect thus detected is delivered as defect data through an OR gate 79.
In the conventional technique described in the above publication, since the mask area must be divided into a large number of sub-areas beforehand, the division itself costs large man-hours. In addition, the judgement section 78 or 80 must determine the threshold for comparison each time the pattern test advances crossing a boundary between the sub-areas, which complicates the control for the comparator in the pattern test device and thus reduces the throughput of the pattern test device.